1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation of the type having an x-ray image intensifier video chain with a video pick-up means coupled through an optic system to a detector for identifying the average image brightness within a predetermined region of the output luminescence screen of the x-ray image intensifier.
2. Description of the Prior Art
In x-ray diagnostics installation of the general type described above, the average image brightness, during an image exposure, is used to control the x-ray generator of the installation, so as to maintain the average brightness at, or within a range of, a specified value. Such an x-ray diagnostics installation is disclosed in U.S. Pat. No. 4,517,594 and European Application 0 362 427, wherein an image is coupled out in the parallel beam path of light distributor, the image being imaged by means of an optical system onto a detector array which is in the form of a matrix. The portion of the image used for controlling the exposure, known as the dominant, is selected with an adjustment means so that output signals of the detector elements selected in this manner are supplied to a comparator, wherein they are compared to a selectable (adjustable) specified value. The comparator controls the high voltage generator which supplies power to the x-ray tube of the x-ray diagnostics installation in accordance with the comparator output. A standard video camera having a video pick-up tube can be used as the video pick-up means, or alternatively a solid state image transducer, as disclosed in German OS 3842649, can be used. In known systems of this type, the necessity of having a light distributor (divider) and an additional detector for the dominant have proven to be disadvantageous.
An x-ray diagnostics installation is disclosed in U.S. Pat. No. 5,012,504 wherein the output signal of a video camera is supplied to the video chain for image processing, and to a control mechanism for the image brightness. A disadvantage of this know installation is that the control signal is only available synchronized with the video clock. A fast control and a chronologically arbitrary disconnect of, for example, the high voltage supply, is therefore not possible.